Means and method of altering the frequency of piezoelectric crystals



Nov, L 1949 H. R MOULTON 2,486,968

MEANS AND METHOD OF ALTERING THE FREQUENCY OF PIEZOELECTRIC CRYSTALS Filed Nov. 22, 194s *4 5 JNVENTOR. /lss HAEOLD le. MouLJ'ON AT TRNEY UNITED lSTATES PATENT OFFICE MEANS AND METHOD or ALTERING 'me FREQUENCY or PmzosLEo'rRlc CRYSTALS Harold R. Moulton,

Southbridge, Mass., assigner to American Optical Company, Southbridge, Mass., a voluntary association of Massachusetts Application November 22, 1943, Serial No. 511,228

22 Claims.

This invention relates to piezoelectric crystals and has particular reference to the provision of novel means and method of controlling the frequencies of said crystals.

One of the principal objects of the invention is to provide novel means and method of piezoelectric crystal fabrication whereby the resultant frequencies of the crystals may be more easily and more accurately controlled.

Fig. 2 is a schematic view illustrating one step of the method embodying the invention;

Fig. 3 is a View generally similar to Fig. 2 of a further step of the method;

Fig. 4 is a sectional view of the crystal having 2 the coating embodying the invention applied thereto; and

Fig. 5 is a diagrammatical view of the crystal and mounting therefor.

It is well known that in order to obtain a piezoelectric crystal which has a given frequency of oscillation that the physical dimensions of the crystal must be very carefully controlled during the fabrication thereof. Such physical dimen- Another object is to provide nove-l means and sions constitute the over all size and thickness method of altering the frequencies of the crystals of the crystal with the said thickness being very whose inherent physical dimensions are such as carefully controlled dimensionally throughout to have higher frequencies than desired whereby the area of the crystal. It is also well known the said frequencies may be lowered a controlled that in order to obtain higher frequencies of amount. 1.-; oscillation the crystals must be reduced in thick- Another object is to provide novel means and ness` It is quite apparent that great care must method of reclaiming crystals whose frequencies be exercised in obtaining the desired uniformity l through manufacturing errors have been raised of thickness in order to obtain the desired freabove the desired value. quencies of oscillation on large scale production.

Another object ls to provide novel means and This has introduced considerable difficulty as a method of altering the frequencies and oscillavery slight reduction of the thickness beyond tions of the crystal with substantially no change that required for a particular frequency of oscilln the inherent stability of said crystals. lation will increase said frequency and thereby Another object is to provide novel means and render the crystal impractical for use for that method of lowering the frequencies of crystals particular frequency. This difficulty becomes with substantially no alteration of the physical more pronounced in the case of high frequency inherent dimensions or shapes of the fabricated crystals such as are used in short wave and ultra crystals. short wave equipment and therefore introduce Another object is to provide novel and simpllmuch more difficulty in producing such crystals ned means and method of more positively conthan in the case of low frequency crystals. Even trolling the frequencies of oscillations of the with crystals having medium frequencies of oscilcrystals thereby greatly facilitating and providlation great care must be taken not to grind too ing ease of fabrication. thin.

Other objects and advantages of the invention One of the prime objects, therefore, of the will become apparent from the following descrippresent invention is to overcome the above diflition taken in conjunction with the accompanyculty by providing novel means and methods of ing drawings and it will be apparent that many changing or altering the frequency of a crystal changes in the details of construction and arfrom that of the inherent frequency of the imrangement of parts and in the steps of the procmediate crystal body in a simple and eflicient esses may be made Without departing from the manner whereby a more positive and flexible invention as expressed in the accompanying control of the frequency may be obtained. claiirrs. I, therefore, do not wish to be limited Referring ymore particularly to the drawings to the exact details of construction, arrangement wherein like characters of reference designate of parts and steps of the method shown and delike parts of the several views, a piezoelectric scribed as the preferred forms only have been crystal 6 such as shown is rst formed to the given by way of illustration. approximate size and thickness required with the Referring to the drawings: said oscillations of said crystal being slightly Fig. 1 is a perspective view of one form of higher than the immediate required frequency of piezoelectric crystal oscillator to which the inoscillation. The crystal 6 is formed in the usual vention is to be applied; manner by known prior art methods and is intended to be used between suitable electrodes l and 8 as shown diagrammatically in Fig. 5. The crystal 6, in order to obtain exactness in the frequency of oscillation, is then coated throughout all of its outer surfaces or throughout one or more of said surfaces to provide a thin film or coating 9 of material thereon which will lower the frequency of oscillation of said crystal. The amount of lowering of the frequency of oscillation depends upon the thickness of coating 9 or the number of coatings applied thereto and the material of the coating. l

The coating' may be applied as shown diagrammatically in Fig. 2 Iby grasping the crystal 6 in a suitable holder l and dipping said crystal into the coating solution I I in a suitable container I2.

It is to be understood that the said coating may be applied to the crystal by spinning, brushing, spraying or any other suitable method by which a substantially uniform coating may be applied.

If it is desired to apply the coating to one of the surfaces of the crystal the other surfaces may be supplied with a soluble or removable coating which is not attacked by the coating solution il.

The crystal 6 after having the proper thickness of coating 9 applied thereto is preferably heated to set the coating. This may [be accomplished by placing the coated crystal 6 within a suitable heat chamber such as shown diagrammatically in Fig. 3. In this particular instance the chamber is provided with a plurality of electrical heating units i3 by which the desired amount of heat may be generated internally of the chamber. It is to be understood that other methods of heat may be used.

The solution ii may be conveniently produced by mixing 45 parts by volume of ethyl alcohol, 45 parts by volume of ethyl acetate, 5 parts per volume of tetra-ethyl-ortho-silicate and after thorough stirring parts of .concentrated hydrochloric acid in aqueous solution. While hydrochloric acid is mentioned other volatile inorganic acids may be used suc/h as hydrobromic acid, or organic acids, such as formic acid, chloracetic acid, etc. may .be used. Other solvents such as butyl alcohol, iso-propyl alcohol, monoethylester-of-monoethylene-glycol and similar water miscible solvents may be used in preparation of different solvents. While the above proportions are shown by way of example to be a good workable solution, it is to be understood that the relative proportion of the various ingredients, whatever solvents be used, may be varied. For examplc, a solution consisting of 85 parts of ethyl alcohol, 5 parts hydrochloric acid and l0 parts 0f tetraethyl-ortho-silicate may be used whereby a thicker coating is obtained. If a thinner coating is desired the content of the most active ingredient, namely, tetra-ethyl-ortho-silicate may be reduced. To facilitate application other solvents as described aibove may be used and in such cases the content of tetra-ethyl-ortho-silicate would be modified in order to provide a coating of the desired thickness.

A similar coating of a titanium compound may be produced by applying in a similar manner a coating forming solution made by the addition of titanium tetra-chloride to ethyl alcohol. Other alcohols may be used in place of the ethyl alcohol. Other alcohols may be vused and other solvents mixed therewith, before, during or after the addition of titanium tetra-chloride.

The lower order alcohols, f methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, secondary butyl alcohol, the water miscible Cellosolves and their derivatives, water, l-4-dioxane, or in general water miscible organic solvents may be used, the proportion being dependent upon the conditions of application. An example of a solution which has |been found to produce good results is made by adding slowly with stirring 5 grains of titanium tetrachloride to grains of ethyl alcohol. The addition must be made slowly because the reaction is rather violent. A pale yellow liquid results which when applied in a thin layer and dried deposits a thin, coherent, hard, durable and rigid layer consisting substantially of titanium oxide which like the silicon containing layer above described lowers the frequency of the piezoelectric crystal to which it has been applied, the extent of lowering depending upon the thickness of the coating and the number of coatings applied.

Other suitable decomposable compounds of titanium such as titanium tetra-bromide may be used with substantially the same solvents as used in Tables III and IV and in substantially the same proportions. Composite solutions containing both tetraethyl-orthosilicate and titanium tetra-chloride may be used, whereby a layer or coating containing intermingled silicon dioxide and titanium dioxide in any desired proportions may be produced.

Table I Parts by weight Ethyl alcohol 0-90 Ethyl acetate 0-50 Tetra-ethyl-ortho-silicate 0.1-10 Hydrochloric acid 0.1-10 Butyl alcohol 0-90 Monoethyl ether of mono-ethylene glycol 0-90 Table II Parts by weight Volatile water miscible organic vehicle Up to 99.8 Tetra-ethyl-ortho-silicate 0.1 to 10 Acid 0.1 to 10 Table III Parts by weight Ethyl alcohol 0 to 99.9 Titanium tetra-chloride 0.1 to 20 Volatile, water miscible organic vehicle 0 to 90 Tetra-ethyl-ortho-silicate 0.1 to 10 Acid 0 to 10 Table I shows proportions of various ingredients which have given useful results by the deposition of a silicon dioxide layer.

Table II shows, more generally, the proportions of volatile, water miscible organic vehicle, tetraethyl-ortho-silicate, and acid which have been found useful in producing solution-deposited silicon dioxide layers.

Table III shows the proportions of various ingredients which have given useful results by the solution deposition of a titanium dioxide layer.

Table IV shows the proportions of various ingredients which have been found useful in solution depositing layers containing both silicon dioxide and titanium dioxide.v

It is to be understood that the invention herein disclosed is particularly adaptable to reclaiming crystals which have been erroneously reduced to a smaller dimension than desired for obtaining a particular frequency of oscillation, but if it is desired to obviate the extreme accuracy of fabrication in forming such piezoelectric crystals the said crystals may be reduced slightly beyond the dimension required and may thereafter be made to oscillate at the required crystal frequency by the coating treatment above described.

The above provides a simple and emcient and extremely accurate method of obtaining the required frequency of oscillation.

Crystals coated in the above manner are permanently resistant to weathering and mechanically stable.

The piezoelectric crystals referred to herein may be any of the co-mmonly known type such as quartz, tourmaline, etc.

From the foregoing description it will be seen that simple, economical and efficient means have been used to accomplish all of the objects and advantages of the invention.

Having described my invention, I claim:

1. A piezoelectric crystal having upon a surface thereof a thin adherent layer of silicon dioxide of a controlled thickness, said thickness being proportionate to the frequency of oscillation of the crystal desired.

2. A piezoelectric crystal having upon a surface thereof a solution deposited thin adherent layer consisting substantially of silicon dioxide of a controlled thickness, said thickness being proportionate to the frequency of oscillation of 'the crystal desired.

3. A piezoelectric crystal having upon a surface thereof a thin adherent iilm or layer of silicon compound of a controlled thickness, said thickness being proportionate to the frequency of oscillation of the crystal desired.

4. A piezoelectric crystal having upon a surface thereof a solution deposited thin adherent layer of silicon compound of a controlled thickness, said thickness being proportionate to the frequency of oscillation of the crystal desired.

5. A piezoelectric crystal having upon a surface thereof a thin adherent layer of titanium dioxide of a controlled thickness, said thickness being proportionate to the frequency of Oscillation of the crystal desired.

6. A piezoelectric crystal having upon a surface hereof a solution deposited thin adherent f layer of titanium compound of a controlled thickness, said thickness being proportionate to the frequency of oscillation of the crystal desired.

7. A piezoelectric crystal having upon a surface thereof a thin adherent layer oftitanium dioxide and silicon dioxide of a controlled thickness, said thickness being proportionate to the frequency of oscillation of the crystal desired.

8. A piezoelectric crystal having upon a surface thereof a solution deposited thin adherent layer containing titaniumdioxide and silicon dioxide of a controlled thickness, said thickness being proportionate to the frequency of oscillation of the crystal desired.

9. A piezoelectric crystal having upon a surface thereof a thin adherent layer resulting from the coating of said surface with a solution containing:

Monoethyl ether of monoethylene glycol.- 0-90 and with said layer being of a controlled thickness proportionate to the desired frequency of oscillation of the crystal.

10. A piezoelectric crystal having upon a surface thereof a thin adherent layer resulting from the coating of said surface with a solution containing:

Parts by weight Volatile water miscible organic vehicle Up to 99.8 Tetra-ethyi-ortho-silicate 0.1 to 10 Acid 0.1 to 10 and with said layer being of a controlled thickness proportionate to the desired frequency of oscillation of the crystal.

11. A piezoelectric crystal having upon a surface thereof a thin adherent layer resulting from the coating of said surface with a solution containing:

Parts by -weight Ethyl alcohol 0 to 99.9 Titanium tetra-chloride 0.1 to 20 Methyl alcohol 0 to 99.9 Butyl alcollol 0 to 20 Monoethyl etherof monoethylene glycol 0 to 20 and with said layer being of a controlled thickness proportionate to the desired frequency of oscillation of the crystal.

l2. Apiezoelectric crystal having upon a sur. face thereof a thin adherent layer resulting from the coating of said surface with a solution containing:

Ethyl alcohol 0 to 99.8 Titanium tetra-chloride 0.1 to 10 Volatile, water miscible organic vehicle il to Tetra-ethyl-ortho-silicate 0.1 to 10 Aleid 0 to 10 and with said layer beingof a controlled thickness proportionate to the desired frequency of oscillation of the crystal.

13. The method of modifying the natural frequency of oscillation of a piezoelectric crystal comprising depositing a coating from a solution containing:

Parts by weight Ethyl alcohol 9-90 Ethyl acetate 0-50 Tetra-ethyl-ortho-silicate- 0.1-10 Hydrochloric acid 0.1-10 Butyl alcohol 9-90 Monoethyl ether of mono-ethylene glycol 0-90 cle Up to 99.8 Tetra-ethyl-ortho-silicate 0.1 to 10 Acid 0.1 to 10 controlling the thickness of the coating in proportion to the desired frequency of oscillation of the crystal, and thereafter heating said coated article to set the coating.

15, The method of modifying the natural frequency of oscillation of a piezoelectric crystal comprising depositing a coating from a. solution containing:

Parts by weight Ethyl alcohol l to 99.9 Titanium tetra-chloride 0.1 to 20 Methyl alcoholl 0 to 99.9 Butyl alcohol 0 to 20 Monoethyl ether of monoethylene glycol 0 to 20 Parts by weight Ethyl alcohol 0 to 99.8 Titanium tetra-chloride 0.1 to Volatile, Water miscble organic vehicle O to 90 Tetra-ethyl-ortho-silicate 0.1 to 10 Acid 0 to 10 controlling the thickness of the coating in proportion to the desired frequency of oscillation of the crystal, and thereafter heating said coated article to set the coating.

17. The method of modifying the natural frequency of oscillation of a piezoelectric crystal comprising depositing a thin adherent coating of silicon dioxide and controlling the thickness of the coating in proportion to the frequency of oscillation desired.

18. A piezoelectric crystal having upon a surface thereof a thin adherent layer resulting from the coating of said surface with a solution containing a volatile, water miscible organic vehicle and a decomposable material selected from the group consisting of tetraethylorthosilicate, titanium tetrachloride, and mixtures thereof, the

evaporation of the vehicle from the solution and portionate to the frequency of oscillation of theI article desired.

19. A piezo-electric crystal comprising a substrate of piezo-electric material of a given frequency of oscillation greater than required for the resultant crystal, and a thin firmly-adherent layer on the surface of said substrate of a material which will reduce the frequency of oscillation of the substrate, said layer consisting substantially of a material selected from the group consisting of silicon compound, titanium compound and mixtures of silicon compound and titanium compound, the thickness of said layer being sufficient to reduce the frequency of oscillation of the substrate to the frequency desired for the resultant crystal.

20. A piezo-electric crystal comprising a substrate of piezo-electric material of a given frequency of oscillation greater than required for the resultant crystal, and a, firmly adherent layer on the surface of said substrate of a material which will reduce the frequency of oscillation oi the substrate, saidlayer consisting substantially 3 of a material selected from the group consisting of silicon dioxide. titanium dioxide and mixtures of silicon dioxide and titanium dioxide, the thickness of said layer being sufficient to reduce the frequency of oscillation of the substrate rto the frequency desired for the resultant crystal.

21. 'I'he method of modifying the frequency of oscillation of a piezo-electric crystal comprising the steps of applying and forming on the surface of the piezo-electric crystal, said crystal having been initially cut to dimensions having -a frequency of oscillation in excess of that desired for the final crystal, a liquid layer consisting of a watermiscible Volatile organic solvent containing a controlled amount of a material that on decomposition will produce an oxide selected from the group consisting of silicon dioxide, titanium dioxide and mixtures of silicon and titanium dioxide, the amount of the decomposable material in said liquid layer being controlled according to the thickness of the oxide layer desired on evaporation of the solvent, and evaporating the solvent from said liquid layer so as :to produce a dry layer of the oxide and with the thickness of said oxide layer being suiiicient to reduce the frequency of oscillation of the crystal to the frequency desired for the coated crystal.

22. The method of modifying the frequency of oscillation of -a piezo-electric crystal comprising the steps of applying to and forming on the surface of the Piezo-electric crystal, said crystal having` been initially cut to dimensions having a frequency of oscillation in excess of that desired for the nal crystal, a liquid layer consisting of a water-miscible volatile organic solvent containing a controlled amount 0f decomposable material selected from the group consisting of tetraethylorthosilicate, titanium tetrachloride and mixtures thereof, the amount of the decomposable material in said liquid layer being controlled according to the thickness of the resultant layer desired on evaporation of the solvent, and evaporating the solvent from said liquid layer so as to produce a dry layer of the decomposition product of said decomposable material and with the thickness of said decomposition product being suffi- Y cient to reduce the frequency of oscillation of the crystal rto the frequency desired for the coated crystal.

HAROLD R. MOULTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,692,074 Burtls Nov. 20, 1928 1,848,630 Hulburt Mar. 8, 1932 1,995,257 Sawyer Mar. 19, 1935 2,078,229 Bokovoy et a1 K..- Apr. 27, 1937 2,101,893 Bokovoy et al. Dec. 14, 1937 2,402,515 Wainer June 18, 194( FOREIGN PATENTS I Number Country Date 18,616 Australia Feb. 26, 1922 

